1. Field of the Invention
The present invention relates to a liquid crystal display apparatus which can implement high transmittance, high-speed response and a wide viewing angle. More specifically, the present invention relates to a liquid crystal display apparatus where a liquid crystal is sealed in between two substrates, and the manufacturing method therefor, and particularly to a liquid crystal display apparatus where a liquid crystal, to which a polymerizable compound (polymer component) is added, is sealed between two substrates, then an alignment control layer or polymer network is formed by polymerizing the polymerizable compound, and the manufacturing method therefor. Also the present invention relates to a high image quality liquid crystal display apparatus which has characteristics in the movement mode of the liquid crystal.
2. Description of the Related Art
Recently liquid crystal display apparatus are being used in a variety of applications because of such features as thinness, light weight, low voltage driving and low power consumption. The display characteristics are now equivalent to those of a CRT, and are therefore used for such applications as monitors or TVs, where conventionally CRTs have been primarily used.
A general liquid crystal display apparatus has a structure where a liquid crystal is sealed between two substrates. On one of the substrates, thin film transistors (hereafter called TFT) and pixel electrodes are formed, and on the other substrate, a common electrode, color filters, etc. are formed. Hereafter a substrate where TFTs and pixel electrodes are formed is called a TFT substrate, and the substrate which faces the TFT substrate is called a counter substrate.
As a liquid crystal display apparatus which has superb viewing angle characteristics and contrast characteristics, an MVA (Mult-domain Vertical Alignment) liquid crystal display apparatus has been known (e.g. Japanese Patent No. 2947350). In an MVA liquid crystal display apparatus, a liquid crystal aligns in the vertical direction with respect to the substrate surface when no voltage is applied, and the orientation to which a liquid crystal is tilted is regulated by bumps or dents formed on the substrate face or by slits formed on electrodes, when voltage is applied.
FIG. 1 shows an example of a patterned pixel electrodes structure in an MVA liquid crystal display apparatus. This pixel electrode is comprised of a cross-shaped main region 1 and four branch regions which linearly extend in 45°, 135°, 225° and 315° orientations. In a branch section, the width of the electrode sections and the slit sections are about 3 μm, respectively. The electrode on the substrate facing this (not illustrated) is a one-plane electrode which is uniform on the entire surface.
If voltage is applied to electrodes where fine slits are formed, as shown in FIG. 1, a liquid crystal tends to tilt in the directions along the slits. In the case of FIG. 1, if voltage is applied, liquid crystal molecules 4 in the neighboring area 3 of the main region start to tilt along the slit directions as illustrated, the behavior of the liquid crystal molecules propagate to the liquid crystal molecules in the branch regions, which sequentially tilt along the slit directions. As a result, the liquid crystal layer forms a pattern according to the pattern provided by the electrodes which exist outside, and a four-domain alignment, where liquid crystal molecules tilt in four orientations (directions) respectively in the four branch regions, is implemented.
However, it takes time for all the liquid crystal molecules to finally tilt, since the behavior of the liquid crystal molecules near the main region propagate into the peripheral area when voltage is applied. Also if the branch region is long, liquid crystal molecules may tilt in the orientation B, that is the opposite of the proper orientation A to which the liquid crystal molecules are supposed to tilt, in a branch region away from the neighboring area of the main region. This is probably because the liquid crystal molecules tilt before the behavior of the liquid crystal molecules of the neighboring area of the main region propagate into the peripheral area. In this case, a boundary region is formed between A and B, and causes a drop in light transmittance since this boundary region does not transmit light when voltage is applied.
As a means of solving the above problem, a method of applying voltage on the liquid crystal layer, which has been formed by sealing a liquid crystal composition comprising a liquid crystal and a polymerizable compound so as to control the alignment, and irradiating active energy rays onto the substrate surface so as to polymerize the compound, has been proposed for MVA liquid crystal display apparatuses (see Japanese Unexamined Patent Application Publication No. H7-43689 (Claims), Japanese Unexamined Patent Application Publication No. H9-146068 (Claims), and Japanese Unexamined Patent Application Publication No. H10-147783 (Claims)).
In the case of sealing a liquid crystal composition, which comprises a liquid crystal and a polymerizable compound, in an MVA type liquid crystal display apparatus which has an electrode pattern shown in FIG. 1, for example, the generation of liquid crystal molecules which tilt in the opposite orientation, as shown in B in FIG. 1, can be prevented by gradually increasing the applied voltage over time when the voltage is applied to implement the four-domain alignment, as described above. So the compound is polymerized by irradiating active energy rays onto the panel face in this status. Then the compound is polymerized and the tilting orientation or tilting direction of the liquid crystal in the voltage applied status is fixed.
In the liquid crystal display apparatus manufactured in this way, the liquid crystal is somewhat inclined (tilted) in the tilting direction with respect to the vertical direction, even when no voltage is applied. Therefore the response speed is improved when voltage is applied, and a uniform and consistent alignment status is implemented. For this type of liquid crystal display apparatus, a liquid crystal display apparatus with high transmittance is implemented because bumps, etc., which cause the drop in transmittance, need not be formed. In other words, in the case of such an MVA liquid crystal apparatus, high transmittance, high-speed response and uniform and consistent alignment status can be implemented compared with conventional MVA type liquid crystal apparatuses.
However in this system, it is necessary to pattern the electrodes to regulate the tilting direction of the liquid crystal, which can be the cause of fluctuation in quality, complicated processing, dropping the yield and increasing cost. Particularly in the case of forming fine slits as shown in FIG. 1, transmittance is changed by a slight fluctuation of patterning, so a very high precision manufacturing process is demanded.
Now the MVA liquid crystal display apparatus will be more fully described. FIG. 2 is a cross-sectional model view depicting an example of a conventional MVA type liquid crystal display apparatus. This MVA liquid crystal display apparatus has a structure where a liquid crystal 70, of which the dielectric constant anisotropy is negative, is sealed between a TFT substrate 50 and a counter substrate 60. Polarizing plates (not illustrated) are disposed respectively under the TFT substrate 50 and on the counter substrate 60 such that the polarization axes are perpendicular to each other.
The TFT substrate 50 is comprised of a glass substrate 51, pixel electrodes 52, which are formed on the liquid crystal 70 side surface (top surface in FIG. 2) of the glass substrate 51, and a vertical alignment control film 53 which covers the surface of the pixel electrodes 52. The pixel electrodes 52 are made of a transparent conductor, such as ITO (Indium-Tin Oxide), and the vertical alignment control film 53 is made of a polyimide, a polyamic acid or the like.
The counter substrate 60, on the other hand, is comprised of a glass substrate 61, a common electrode 62, which is formed on the liquid crystal 70 side surface (bottom surface in FIG. 2) of the glass substrate 61, bumps for domain regulation 63 which are formed on the common electrode 62, and a vertical alignment control film 64 which covers the surface of the common electrode 62 and the bumps 63. The common electrode 62 is made of a transparent conductor such as ITO, and the vertical alignment control film 64 is made of a polyimide, a polyamic acid or the like. The bumps for domain regulation 63 are formed by a photoresist, for example.
FIG. 3A and FIG. 3B are model diagrams depicting the operation of the MVA liquid crystal display apparatus. As FIG. 3A shows, when no voltage is applied between the pixel electrodes 52 and the common electrode 62, liquid crystal molecules 70a align roughly vertically from the surface of the alignment control films 53 and 64. In this status, light, which transmitted through the polarizing plate under the TFT substrate 50 and entered the liquid crystal layer, transmits through the liquid crystal layer as it is, and is shielded by the polarizing plate on the counter substrate 60. In other words, this is the case of a dark display.
On the other hand, if sufficient voltage is applied between the pixel electrodes 52 and the common electrode 62, as shown in FIG. 3B, liquid crystal molecules 70a align in roughly a vertical direction with respect to the electric field. In this status, the light, which transmitted through the polarizing plate under the TFT substrate 50 and entered the liquid crystal layer, is double-refracted because of the dielectric constant anisotropy of the liquid crystal molecules, and transmits through the polarizing plate on the counter substrate 60. In other words, this is the case of a bright display. In this way, by controlling the voltage between the pixel electrodes 52 and the common electrode 62 for each pixel, desired images can be displayed on the liquid crystal display apparatus.
In the case of an MVA liquid crystal display apparatus, if voltage is applied between the pixel electrodes 52 and the common electrode 62, the tilting directions of the liquid crystal molecules differ between each side of a bump 63 as shown in FIG. 3B, and a so called “alignment division (multi-domain)” is formed. By this, the leaking of light in diagonal directions from the substrate face is decreased remarkably, and good viewing angle characteristics and contrast characteristics can be implemented.
The above described example is the case when the bumps 63 are formed as domain regulation means, but the domain regulation means may be implemented by forming slits on one or on both of the pixel electrodes and the common electrode. In a normal MVA liquid crystal display apparatus, slits are formed on the electrodes of one of the TFT substrate and the counter substrate, and bumps are formed on the other substrate. Also dents may be formed instead of bumps or slits as the domain regulation means.
Japanese Unexamined Patent Application Publication No. H7-84244 and Japanese Unexamined Patent Application Publication No. H11-343486 disclose a manufacturing method of a liquid crystal display apparatus where a member for regulating the liquid crystal domains is formed by polymerizing a photoreactive monomer, which is added to the liquid crystal. With the method disclosed in Japanese Unexamined Patent Application Publication No. H11-343486, however, a step of forming a vertical alignment control film is required, just as with conventional methods. And with the method disclosed in Japanese Unexamined Patent Application Publication No. H7-84244, the portions to be the boundary of domains cannot be set at arbitrary positions.
Now a comparison between the MVA liquid crystal display apparatus and the horizontal electric field switching type liquid crystal display apparatus, called “In-Plane” switching (hereafter called IPS), will be described. FIG. 4 is a view depicting an MVA type vertically aligned liquid crystal display apparatus based on the prior art, and FIG. 5 is a view depicting a horizontal electric field switching type liquid crystal display apparatus based on the prior art. The MVA liquid crystal apparatus and IPS liquid crystal apparatus are well known for superb characteristics thereof.